Numerical analysis on effect of exit blade angle on cavitation in centrifu

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

359

NUMERICAL ANALYSIS ON EFFECT OF EXIT BLADE ANGLE ON

CAVITATION IN CENTRIFUGAL PUMP

Shalin Marathe Rishi Saxena

M.E. (CAD/CAM) Assistant Professor

Mechanical Engineering Department Mechanical Engineering Department

Sardar Vallabhbhai Patel Institute Sardar Vallabhbhai Patel Institute

of Technology, VASAD of Technology, VASAD

ABSTRACT

This paper presents the effect of outlet blade angle on cavitation in centrifugal pump.

The experiment is performed on a centrifugal pump test rig consisting of backward bladed

impeller at different operating conditions and characteristics of the pump are predicted.

Modeling of the centrifugal pump along with the different configuration of the impeller

having different exit blade angles is carried out using Creo Parametric. Numerical simulation

is carried out using ANSYS CFX and standard k-� turbulence model is implemented for the

analysis purpose. Cavitation is clearly predicted in the form of water vapor formation inside

the centrifugal pump from the simulation results. Analytical analysis is carried out in order to

find out NPSHr of the pump and Cavitation number (σc) which indicates the cavitation

phenomenon in the centrifugal pump. From the results it has been found that the pump having

low value of the blade exit angle will have less chances of getting affected by the cavitation

phenomenon.

KEY WORDS: ANSYS CFX, Cavitation, Cavitation number, Centrifugal pump, NPSHr,

Numerical Simulation, Turbulence Model k-�.

1 INTRODUCTION

In centrifugal pump, an increase in the fluid pressure from the pump inlet to its outlet

occurs during operation. This pressure difference developed in the pump drives the fluid

through the system. The centrifugal pump creates an increase in pressure by transferring

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mechanical energy from the motor to the fluid through the rotating impeller as shown in

figure 1. Centrifugal pump faces a problem of cavitation. In general, cavitation occurs when

the liquid pressure at a given location is reduced to the vapor pressure of the liquid.

Cavitation begins when the absolute pressure at the inlet falls below the vapor pressure of the

water. This phenomenon may occur at the inlet to a pump and on the impeller blades,

particularly if the pump is mounted above the level in the suction reservoir. Under this

condition, vapor bubbles form at the impeller inlet and when these bubbles are carried into a

zone of higher pressure, they collapse abruptly and hit the vanes of the impeller, near the tips

of the impeller vanes causing damage to the pump impeller, violet vibrations and noise,

reduce pump capacity, reduce pump efficiency.

Figure 1 Centrifugal pump

W.G.Li [1]

stated that the blade discharge angle has a strong but equal influence on the

head, shaft power and efficiency of the centrifugal oil pump for various viscosities of liquids

pumped. The rapid reduction in the hydraulic and mechanical efficiencies is responsible for

the pump performance degradation with increasing viscosity of liquids. E.C.Bacharoudis et al [2]

found that as the outlet blade angle increases the performance curve becomes smoother and

flatter. But M.H.S.Fard et al [3]

stated that pump performance goes down when the pump

handles high viscosity working fluids because high viscosity results in disc friction losses

over outside of the impeller. SHI Weidong et al

[4] investigated that the oversize impeller

outlet width leads to poor pump performances and increasing shaft power. Cavitation also

affect the performance of the pump and it must be avoided. D.Somashekar et al [5]

suggested

that in order to avoid the cavitation available NPSH of the system must be equal to or greater

than the NPSH required by the centrifugal pump and similar recommendations were given by

the M.K.Abbas [6].

A.Stuparu et al [7]

performed numerical investigation of the multiphase

flow inside the storage pump which underlines the fact that the pumping head drops due to

the development of cavitation phenomena. A.Goto et al [8]

found that at the high flow rate

cavitation bubbles appear at the leading edge on pressure side incipiently and the head drops

gradually. J.B.Jonker et al [9]

suggested that cavitation inception for the forward swept

impellers occurs at half-span of the leading-edge, while it occurs close to the shroud for the

backward-swept impeller. Also It has been found that the backward bladed impeller gives the

highest efficiency to the centrifugal pump in compare to the forward and radial bladed

impeller. But the energy transfer is less for the backward bladed impeller in comparisons of

radial and the forward bladed impeller.

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May

2 EXPERIMENTATION

Experiment is carried out

having the backward bladed impeller.

and table 1 shows the specification of

Figure 2 Centrifugal pump test rig

3 MODELING AND SIMULATION

Creo Parametric 1.0 version is used for geometric modeling of the impeller having

different blade angles and the casing. The figure 3 and figure 4 shows the Creo model of the

Impeller and the casing of the centrifugal pump

Figure 3 Creo model of

Table 1 Centrifugal Test rig specification

Pump Total Head

Discharge

Speed

Motor

Measuring Tank

Sump tank


6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME

361

Experiment is carried out at different operating conditions on the centrifugal pump

impeller. The figure 2 shows the test rig of the centrifugal pump

cification of the pump test rig.

Figure 2 Centrifugal pump test rig

SIMULATIONS



Impeller and the casing of the centrifugal pump respectively.

Creo model of impeller

Figure 4 Creo model of casing

Table 1 Centrifugal Test rig specification

Pump Total Head 12 m

Discharge 1.5 lps

Speed 2900 rpm

Motor 1 HP

Measuring Tank 400 400 450 mm Height

Sump tank 600 900 600 mm Height


June (2013) © IAEME

on the centrifugal pump

shows the test rig of the centrifugal pump



Figure 4 Creo model of casing



ANSYS CFX (14.0) is the simulation tool which is used for the numerical analysis of

the centrifugal pump. Meshing is carried out using the Auto Mesh feature of the ANSYS

WORKBENCH, which is shown in the figure 5 which

as the elements and the number of nodes and the elements generated are

respectively during the mesh. To predict the complex behavior of the flows

standard k- model is adopted.

Figure 5 Meshing of the model

One of the major advantages of the ANSYS is that the user can give the boundary

conditions close to the actual operating conditions. Analysis is carried out in a Steady state

condition taking 1 atmospheric as the reference pressure. Rotating velocity is provided to the

impeller along the Z direction. Two fluids namely as Water and Water vapor are selected in

order to determine the formation of vapors inside the pump. At inlet a

conditions are provided and the discharge of the pump is determined from the simulation in

order to match with the experimental.

5 NUMERICAL RESULTS AND

After the completion of the solver part of the simulation, results like

and the water vapor formation contours are generated as shown in the figure for the different

discharge conditions like 1.5, 5 and 10 lps, for all the configurations of the impellers.

(a) 20 degree bladed impeller

(d) 60 degree bladed impeller

Figure 6 Water vapor contours at 1.5 lps



362



which is shown in the figure 5 which indicates that the tetrahedrons a

he number of nodes and the elements generated are 77542 and 419333

respectively during the mesh. To predict the complex behavior of the flows inside the pump

Figure 5 Meshing of the model





order to determine the formation of vapors inside the pump. At inlet and outlet pressure


order to match with the experimental.

AND DISCUSSIONS

After the completion of the solver part of the simulation, results like pressure contours



(b) 30 degree bladed impeller (c) 40 degree bladed

impeller

(e) 70 degree bladed impeller (f) 80 degree bladed

impeller

Figure 6 Water vapor contours at 1.5 lps





indicates that the tetrahedrons are used

77542 and 419333

inside the pump





nd outlet pressure


pressure contours



(c) 40 degree bladed

impeller

(f) 80 degree bladed

impeller



Figure 6 and 8 shows

respectively It indicates that at higher exit blade angle, the volume occupied by the water

vapor is more as compare to lower exit blade angle.

particular blade angle the volume occupied by the water vapor a

lps. As the density of the water is competitiv

are settled on the upper side of the casing and the impeller

Figure 7 and 9 shows the pressure variat

5 lps. On analyzing the pressure contours, higher pressure is observed at the discharge section



Figure 7 Pressure contours at 1.5 lps



Figure 8 Water vapor contour



363

phenomenon of water vapor formation at 1.5 lps

that at higher exit blade angle, the volume occupied by the water

vapor is more as compare to lower exit blade angle. Also thing should be noted that for a

particular blade angle the volume occupied by the water vapor at 5 lps is more than the 1.5

s the density of the water is competitively higher than the water vapor, the

are settled on the upper side of the casing and the impeller.

shows the pressure variation across the centrifugal pump at 1.5 lps and

On analyzing the pressure contours, higher pressure is observed at the discharge section

(b) 30 degree bladed impeller (c) 40 degree bladed impeller

(e) 70 degree bladed impeller (f) 80 degree bladed impeller

Figure 7 Pressure contours at 1.5 lps

(b) 30 degree bladed

impeller

(c) 40 degree bladed

(e) 70 degree bladed

impeller (f) 80 degree bladed impeller

Figure 8 Water vapor contours at 5 lps



ater vapor formation at 1.5 lps and 5 lps

that at higher exit blade angle, the volume occupied by the water

Also thing should be noted that for a

t 5 lps is more than the 1.5

ely higher than the water vapor, the water vapors

ion across the centrifugal pump at 1.5 lps and

On analyzing the pressure contours, higher pressure is observed at the discharge section

c) 40 degree bladed impeller

(f) 80 degree bladed impeller

(c) 40 degree bladed impeller

bladed impeller



on varying the blade angle from 20 degree to 80 degree in both the condition of 1.5 lps and 5

lps. Due to the increased discharge there is no pressure difference in 2

impeller from inlet section to discharge section. T

lps is less than the 1.5 lps for pump

pressure development inside the centrifugal pump as we

lead to the low pressure regions and also to the

in pressure from inlet section to outlet section for all

Since in the impeller section there is no change

recirculation and back flow will continue until the desired pressure is attainable. A low

pressure region is observed near the

(a) 20 degree bladed impeller (b) 30 degree bladed impeller

(d) 60 degree bladed impeller (g)70 degree bladed impeller

Figure 9 Pressure Contour

Figure 10 Result comparison

Figure 10 shows the good

results. Figure 11 shows the effect

Suction Head required). It indicates

0

0.5

1

1.5

2

2.5

0 2 4 6 8

DIS

CH

AR

GE

(lp

s)

EXPERIMENT NUMBER

EXPERIMENTATION

SIMULATION



364

gle from 20 degree to 80 degree in both the condition of 1.5 lps and 5

lps. Due to the increased discharge there is no pressure difference in 20 degree bladed

impeller from inlet section to discharge section. The pressure developed inside the

pump having 20 degree impeller. So there is a decrease in

pressure development inside the centrifugal pump as we increases the discharge. So that may

lead to the low pressure regions and also to the cavitation phenomenon. There is gradual rise

in pressure from inlet section to outlet section for all other configuration of the blade angles.

ion there is no change in pressure, it will lead to the phenomenon of


pressure region is observed near the tongue section of the casing due to formation of eddies.

(b) 30 degree bladed impeller (c) 40 degree bladed impeller

(g)70 degree bladed impeller (h) 80 degree bladed impeller

Figure 9 Pressure Contours at 5 lps

comparison Figure 11NPSHr vs. Discharge

shows the good agreement of the simulation results with the

effect of discharge on to the value of NPSHr

ed). It indicates that as the value of the discharge increases the NPSH

10 12

EXPERIMENT NUMBER

EXPERIMENTATION

0

1

2

3

4

5

6

7

8

0 0.002 0.004

NP

SH

r

Q (DISCHARGE) (m3/sec)



gle from 20 degree to 80 degree in both the condition of 1.5 lps and 5

0 degree bladed

he pressure developed inside the pump at 5

. So there is a decrease in

increases the discharge. So that may

There is gradual rise

configuration of the blade angles.

in pressure, it will lead to the phenomenon of


section of the casing due to formation of eddies.

(c) 40 degree bladed impeller

(h) 80 degree bladed impeller

Figure 11NPSHr vs. Discharge

experimental

(Net Positive

he discharge increases the NPSHr

0.006



will also increase and that will lead to the cavitation phenomenon. So all the pump should be

operated with in its range of design flow

discharge conditions on to the cavitation

the cavitation number increases and that is nothing but the indication of cavitation

phenomenon. Also with increase in the cavitation number the head of the pump will decrease

along with the performance. For all the configurations of the blade exit angles similar

behavior is observed.

Figure 12 Cavitation number vs. Discharge For Different blade

CONCLUSIONS

It can be concluded from the results that the use of impeller

angle is much efficient than the impeller having higher exit blade angle because the

phenomenon of the cavitation is identified for higher exit blade angles in the

obtained from the simulation. Since the cavitation phenomenon

will lead to the erosion of the impeller material, that will reduce

head in a drastic manner. Hence, if we use the blade angle in a range of 20 degree to 30

degree it provides efficient conditions

REFERENCES

[1] Wen-Guang LI “Blade Exit Angle Effects on Performance of a Standard Industrial

Centrifugal Oil Pump”- Department Of Civil And Environmental Engineering Cvng

1001: Mechanics Of Fluids.

[2] E.C. Bacharoudis, A.E. Filios, M.D. Mentzos And D.P. Margaris

Of A Centrifugal Pump Impeller By Varying The Outlet Blade Angle” In The Open

Mechanical Engineering Journal, 2008, 2, 75

[3] M.H.S.Fard And F.A.Bhoyaghchi

Outlet Angles In A Centrifugal Pump When Handling Viscous Fluid" In American

Journal Of Applied Science 2007.

-5

0

5

10

15

20

25

30

35

0 2

CA

VIT

AT

ION

NU

MB

ER



365

l lead to the cavitation phenomenon. So all the pump should be

operated with in its range of design flow rate. Figure 12 shows the effect of different

cavitation number which shows that as the discharge increases


Also with increase in the cavitation number the head of the pump will decrease

e performance. For all the configurations of the blade exit angles similar


configuration

It can be concluded from the results that the use of impeller having low exit blade


phenomenon of the cavitation is identified for higher exit blade angles in the

obtained from the simulation. Since the cavitation phenomenon is undesirable for the pump it

will lead to the erosion of the impeller material, that will reduce the efficiency, discharge,

in a drastic manner. Hence, if we use the blade angle in a range of 20 degree to 30

degree it provides efficient conditions for operating the centrifugal pump.

Guang LI “Blade Exit Angle Effects on Performance of a Standard Industrial

Department Of Civil And Environmental Engineering Cvng

1001: Mechanics Of Fluids.

, A.E. Filios, M.D. Mentzos And D.P. Margaris- " Parametric Study


Mechanical Engineering Journal, 2008, 2, 75-83.

M.H.S.Fard And F.A.Bhoyaghchi –” Studies On The Influence Of The Various Blade


Journal Of Applied Science 2007.

4 6 8 10 12

DISCHARGE (LPS)

20 DEGREE

30 DEGREE

40 DEGREE

60 DEGREE

70 DEGREE

80 DEGREE



l lead to the cavitation phenomenon. So all the pump should be

effect of different

number which shows that as the discharge increases


Also with increase in the cavitation number the head of the pump will decrease

e performance. For all the configurations of the blade exit angles similar


having low exit blade


phenomenon of the cavitation is identified for higher exit blade angles in the contours

is undesirable for the pump it

the efficiency, discharge,

in a drastic manner. Hence, if we use the blade angle in a range of 20 degree to 30

Guang LI “Blade Exit Angle Effects on Performance of a Standard Industrial

Department Of Civil And Environmental Engineering Cvng

" Parametric Study


f The Various Blade


20 DEGREE

30 DEGREE

40 DEGREE

60 DEGREE

70 DEGREE

80 DEGREE



366

[4] SHI Weidong, ZHOU Ling*, LU Weigang, PEI Bing, and LANG Tao-" Numerical

Prediction and Performance Experiment in a Deep well Centrifugal Pump with

Different Impeller Outlet Width"- CHINESE JOURNAL OF MECHANICAL

ENGINEERING Vol. 26,No.-2013

[5] Mr. D. Somashekar1, Dr. H. R. Purushothama –”Numerical Simulation Of Cavitation

Hysteresis On Radial Flow Pump" In IOSR Journal Of Mechanical And Civil

Engineering (Iosrjmce) Issn : 2278-1684 Volume 1, Issue 5 (July-august 2012), Pp 21-

26 Www.iosrjournals.Org

[6] Stuparu, Romeo Susan-resiga, Liviu Eugen Anton And Sebastian Muntean-" A New

Approach In Numerical Assessment Of The Cavitation Behavior Of Centrifugal

Pumps” In International Journal Of Fluid Machinery And Systems Vol. 4, No. 1,

January-march 2011.

[7] Motohiko Nohmi , A. Goto – “Cavitation CFD In A Centrifugal Pump” Fifth

International Symposium On Cavitation (Cav2003),osaka, Japan, November 1-4, 2003.

[8] J.B. Jonker, M. J. Van Os , J. G.H. Op De Woerd "A Parametric Study Of The

Cavitation Inception Behavior Of A Mixed-flow Pump Impeller Using A Three-

dimensional Potential Flow Model" In The 1997 ASME Fluids Engineering Division

Summer Meeting Fedsm’97 June 22–26, 1997.

[9] Manish Dadhich, Dharmendra Hariyani and Tarun Singh, “Flow Simulation (CFD) &

Fatigue Analysis (Fea) of a Centrifugal Pump”, International Journal of Mechanical

Engineering & Technology (IJMET), Volume 3, Issue 3, 2012, pp. 67 - 83, ISSN Print:

0976 – 6340, ISSN Online: 0976 – 6359.

[11] V. Muralidharan, V.Sugumaran and Gaurav Pandey, “Svm Based Fault Diagnosis of

Monoblock Centrifugal Pump using Stationary Wavelet Features”, International

Journal of Design and Manufacturing Technology (IJDMT), Volume 2, Issue 1, 2011,

pp. 1 - 6, ISSN Print: 0976 – 6995, ISSN Online: 0976 – 7002.

[12] V. Muralidharan, V. Sugumaran, P. Shanmugam and K. Sivanathan, “Artificial Neural

Network Based Classification for Monoblock Centrifugal Pump using Wavelet

Analysis”, International Journal of Mechanical Engineering & Technology (IJMET),

Volume 1, Issue 1, 2010, pp. 28 - 37, ISSN Print: 0976 – 6340, ISSN Online:

0976 – 6359.